Roles of corn starch and gellan gum in changing of uncon ﬁ ned compressive strength of Shanghai alluvial clay

: This article aims to study the e ﬀ ects of corn starch and gellan gum (gellan glue) on the uncon ﬁ ned compressive strength of alluvial soft clay in Shanghai. Firstly, starch and gellan gum were evenly mixed into Shanghai cohesive soil in a certain proportion, and soil samples were prepared according to the optimal water content. Secondly, the uncon ﬁ ned compressive strength test was carried out according to the content of corn starch, gellan glue, and curing time. The strength characteristics of Shanghai soft clay under various test conditions were studied. Finally, the software for statistics and drawing was used to perform regression analysis and drawing on the experimental data. The results showed that in the uncon-ﬁ ned compressive strength test, when the curing time was 28 days, the starch content was 3% mixed with 0.5% gellan gum, the uncon ﬁ ned compressive strength of the sample was the highest, and generally, it seems that the strength keep increasing with curing time.


Introduction
Shanghai is located in the center of the alluvial plain of the Yangtze River Delta, where the soil quality is characterized by high water content, low strength, and poor water permeability.It generally cannot meet the requirements of engineering construction, so some measures need to be taken to strengthen it before construction.Traditional physical and chemical reinforcement methods do not conform to the concept of green and sustainable development due to environmental pollution or high cost, so it is urgent to find new reinforcement methods that are nonpolluting to the environment, low cost, and in line with sustainable development.Guodong [1] used the strong compaction method to strengthen the cohesive soil added to the crushed stone mixture and achieved good results.Qi [2] made use of the soil replacement cushion method to replace part or all of the soft clay on the bottom of the foundation, which enhances the bearing capacity of the foundation.Jinzhang and Hongbo [3] adopted the drainage consolidation method to remove pore water from the soil and improve the bearing capacity of the foundation.Guolin [4] used willow branches, wheat straw, rice husks, and other plant fibers to reinforce the ancient city wall.Zhanfei et al. [5] showed that the cohesion of red clay reinforced by polypropylene fiber first increased and then decreased with the increase in fiber length.With the increase in fiber content, the overall "bimodal type" is presented.Lintao et al. [6] used textile fibers prepared from waste clothing to reinforce the clay, and the results showed that when the fiber content reached 1%, the shear strength reached the maximum, and the California bearing ratio (CBR) was also greatly improved compared with plain soil.Guangxin et al. [7] studied the mechanical properties of polypropylene and polyester fiber-reinforced cohesive soil, and the results showed that its bearing capacity and cohesion increased, while the lime-treated soil samples peaked at 0.1% fiber content.Abbaspour et al. [8] showed that waste tire textile fiber can effectively improve the strength characteristics of sand and improve the tensile strength and ductility of sand.Bao et al. [9] studied carbon fiber as a reinforced material to reinforce clay, showing that the soil strength reached the maximum at 3.0% fiber content and 6 mm fiber length, and there is a decrease in shear strength with the increase of carbon fiber content, mainly because carbon fiber could not be evenly distributed in the soil and agglomerated into blocks, reducing the cohesion of the interface.Quang et al. [10] studied the influence of corn silk as a reinforced material on the mechanical properties of soil, showing that corn silk fiber can improve the compressive strength and tensile strength of soil, and the ductility and toughness are also improved, which is an excellent fiber material for improving soil.By studying the effects of polyester fibers of different lengths and contents on the shear strength of expansive soil, Parrihar et al. [11] showed that the content of 0.75% polyester fiber could significantly improve the strength of the soil, and the peak strength of reinforced soil increased by 10-12%.Meddah and Merzoug [12] randomly incorporated rubber fibers into sand, and the results showed that rubber fibers could improve the strength characteristics of sand, improve the shear strength and residual stress of sand, and improve ductility.
Lihua et al. [13] found that PAM (polyacrylamide) can promote the formation of water-stable aggregates in black soil, loess soil, and aeolian sand.Jiayu et al. [14] found that xanthan gum biopolymer improves sand, showing that the unconfined compressive strength and shear strength of the improved soil are significantly improved.Tao and Jili [15] found that xanthan gum improved the erosion resistance of soil, and 1.5% xanthan gum improving soil have the largest compressive strength, and excessive xanthan gum would weaken the strength of clay.Tao et al. [16] found that lignin increased the maximum dry density of silty clay and decreased the optimal moisture content, and 12% was the optimal reinforcement rate.Khatami and O'Kelly [17] used agar and modified starch as reinforcement materials to reinforce the sand, and it was found that agar and modified starch not only improved the unconfined compressive strength of sand but also significantly improved the cohesion and stiffness of sand.Liu et al. [18] studied sulfur-free lignin-modified silty clay with different contents and found that the compressive strength of 10% modified soil reached 1,165 kPa, which was about five times higher than that of plain soil.Jili and Zhongming [19] found that lime powder significantly improved the compressive strength and shear strength of soil, and the strength reached the maximum when the lime powder content was 5%.Bell [20] showed that cement clinker products can improve soil workability, increase soil strength, and are better materials for reinforcing clay foundations.Sabry et al. [21] studied the expansive soil after lime treatment and found that the strength increased with the increase of curing age, but the brittle damage was obvious, and once the soil was disturbed, the strength was rapidly lost.Jianwen et al. [22] added industrial waste phosphogypsum to high moisture content dredged sludge for compression test and found that industrial waste phosphogypsum can solidify the strength of soil, and it increases with the increase of material reinforcement rate.Prabakar et al. [23] studied the effects of different amounts of fly ash on the engineering properties of three types of soils and found that fly ash can improve the shear strength and bearing capacity of the soil and reduce the dry density and expansion rate of the reinforced soil.Zhimin and Jili [24] reinforced Shanghai clay by xanthan gum and palm fiber and found that both xanthan gum and palm silk fiber could improve the mechanical properties and compressive strength of Shanghai clay, and palm silk fiber also improved the ductility of the soil.Huda et al. [25] found that cellulose fiber can increase the maximum dry density of Shanghai clay and reduce its optimal moisture content, and the compressive strength and shear strength of Shanghai clay are also significantly improved.Jili et al. [26] studied palm fiber-reinforced Shanghai clay and found that palm fiber reinforcement significantly improved the shear strength and cohesion of the soil, and improved the resistance to deformation.Yun [27] studied the effect of modified starch on the strength of the plain soil through the unconfined compressive test and the direct shear test, and the results showed that the modified starch could significantly improve the shear strength and compressive strength of the plain soil, and the compressive strength gradually decreased with the increase of starch content, and the shear strength remained basically unchanged.When the starch content is 3%, the compressive strength is maximum.Alhaik et al. [28] studied the effects of starch on thixotropy and mechanical properties of clay through thixotropic test and mechanical test and found that most starch can increase the thixotropic index of ore powder and reduce the thixotropic index of kaolinite.Starch can improve the mechanical properties of clay.Akindahunsi [29] studied the effects of tapioca starch and corn starch on the properties of concrete, and the results showed that both starches could improve the compressive strength of concrete, and starch helped slow creep and shrinkage of concrete.Zhang et al. [30] and Peschard et al. [31] showed that starch derivatives have good dispersion stability and reduce the flow loss of concrete.Schmidt et al. [32] found that starch changes the rheological properties of concrete by increasing the plastic viscosity of concrete, and the yield stress of concrete increases with the increase of solid particle volume fraction, and starch also affects the hydration characteristics of cement and the performance of efficient water-reducing agent.Khatami and O'Kelly [17] have shown that adding starch at the same agar concentration can significantly increase the cohesion of sand.
With the continuous improvement of the awareness of environmental protection and the increasing requirements for the material, cost, and availability for reinforced soil, the author used corn starch and gellan gum to mix Shanghai cohesive soil in different proportions to study its effect on the unconfined compressive strength.The innovation of this article lies in the fact that corn starch and gellan gum are two new materials to improve Shanghai clay, which broaden the categories of reinforcement materials and enrich the theory of combining materials to improve Shanghai clay.In addition, corn starch and gellan gum have the advantages of easy access, natural pollution free, renewable, and comparatively low price.At present, gellan glue is mostly concentrated in industry and food, and there are not many applications in soil reinforcement.The gellan glue is used as the research object to improve the strength of Shanghai clay, enriching the research scope of gellan glue and increasing the selectivity of local engineering construction.

Test soil
The experimental soil was taken from the construction site of No. 516 Jungong Road, Yangpu District, Shanghai, and is representative because it is located on the southern side of the alluvial plain of the Yangtze River Delta, near the Huangpu River, as shown in Figure 1.First, the collected clay is crushed and dried in an oven at a temperature of 105°C for 8 h [ASTM D2487], and after drying, it is crushed with a soil crusher, and then it is screened with a 2 mm standard sieve, and the test soil is shown in Figure 2(a).Through the boundary moisture content experiment and the compaction test, the relevant physical and hydraulic indicators of Shanghai clay were obtained, as shown in Table 1.The JHY5100-H laser particle size analyzer was used for particle size analysis, and the results are shown in Figure 3.The chemical composition of the test soil is shown in Table 2.

Starch
The starch selected for this test is corn starch, which is produced by Shanghai Fengwei Industrial Co., Ltd.The appearance of corn starch for the test is a white powder with a slight yellowish tinge, as shown in Figure 2(b).The main components of the test starch are shown in Table 3. Corn starch is made by crushing, sifting, precipitating, drying, grinding, and other processes, mainly used in food processing, as well as industrial production of starch sugar, amino acids, modified starch, medical antibiotics, food additives, beer, papermaking, etc. Cornstarch is a good filler and binder, and its cost has dropped significantly in China due to industrialization and large-scale production, and its use for soil reinforcement is a bold attempt.its hydrolysis and digestion effect are relatively slow, with swelling resistance, poor water solubility, and good filmforming and strength.Amylose can also be used as a thickener and a gelling agent, as well as replacing polystyrene to produce degradable plastics, with excellent transparency, flexibility, tensile strength, far-reaching significance for solving white pollution and protecting the environment, and has broad application prospects.

Gellan gum
The gellan gum selected for this test was obtained from Henan Wanbang Industrial Co., Ltd., and the appearance is light yellow powder, as shown in Figure 2(c).The low acyl gellan gum is selected in the test, and the molecular structure is shown in Figure 5. Low acyl gellan gum can also form a gel at a low concentration of 0.05-0.25%,which is an efficient gelling agent.Gellan gum, also known as Kecco gum, is a polymer linear polysaccharide, which is repeatedly polymerized by the basic unit composed of four monosaccharide molecules.Its basic unit is composed of 1,3-and 1,4-linked 2 glucose residues, 1,3-linked 1 glucuronic acid residue, and 1,4-linked 1 rhamnose residue.Gellan gum decomposes at about 150°C without melting with high heat resistance, acid resistance, and enzyme stability.Gellan gum, in the presence of cations, produces a hard and brittle gel when heated and cooled.Gellan gum can be used as a thickener and stabilizer.Although it is insoluble in cold water, it is dispersed in water with a little stirring.Upon heating, it dissolves into a transparent solution and after cooling forms a transparent and solid gel.The dosage is small and generally 0.05% can form a gel.As a microbial metabolic glue, gellan glue has a short production cycle, is not limited by climate and geographical environmental conditions, and can be produced under artificial control conditions using various waste residues and waste liquids, coupled with its excellent characteristics such as safety and nontoxicity, unique physical, and chemical properties, and has been widely used in the food industry.This project applies it to the reinforcement of Shanghai cohesive soil, which is of great significance to enrich the selectability of clay soil reinforcement.Table 4 presents the composition and properties of gellan glue used in this test.

Limit water content and optimal water content
This test is used to test the basic hydrology indicators of natural soil.The instrument used for the limit water   content is STYS-1 digital display liquid and plastic limit joint tester, produced by Zhejiang Geotechnical Instrument Manufacturing Company.The cup height is 40 mm, the inner diameter is 40 mm, the conical instrument is 76 g, and the cone angle is 30°.The optimal water content and maximum dry density of natural soil were tested by the compaction test, and the I-1 light compaction instrument was selected for the test, which was produced by Zhejiang Geotechnical Instrument Manufacturing Co., Ltd.The compaction instrument has a diameter of 5 cm, a drop height of 30 cm, an inner diameter of 10 cm, a height of 12.7 cm, and a volume of 997 cm 3 .The test soil was screened by 2 mm.Soil samples are prepared according to the American standard ASTM.After the samples are made, they are put into a constant temperature and humidity curing box for maintenance.The room temperature is maintained at 25 ± 5°C, and the humidity is maintained at 70 ± 5%.

Unconfined compressive strength (UCS) test
The instrument used in the unconfined compressive strength test is WDW-Y300D microcomputer-controlled automatic pressure testing machine, produced by Jinan Zhongzheng Testing Machine Company, with a maximum pressure of 300 kN.The axial pressure speed of the specimen is 2 mm/min.The specimen is cylindrical with dimensions of 39 mm diameter × 80 mm height.

Test protocol 2.5.1 Test technical route
Figure 6 shows the technical roadmap of this experiment.UCS stands for Unconfined Compressive Strength test.

Specimen preparation
1. Soil mass Take a sufficient amount of dry soil that has passed through a 2 mm sieve, weigh it, and gently pour it into the test basin to avoid dust as much as possible.Tap water is added at 21% water content.The optimal moisture content of the soil is 20%, which is actually calculated at 21% considering the evaporation and loss of water during the preparation of the specimen.Slowly add tap water to the soil and gently stir with a stirring bar, and after the water is added, stir for another 10 min to evenly distribute the soil and water.Then put the soil into a constant temperature and humidity box and let it stand for 24 h to fully and evenly integrate the water into the soil.After that, according to the optimal moisture content and maximum dry density of the diameter of 39 × height of 80 mm cylindrical soil volume required for, according to the maximum dry density of 1.68 g/cm 3 , the corresponding weight is calculated.Weigh the soil and put it into a three-  lobe mold in three layers, and the inner diameter of the three-lobe mold is 39 mm.The bulking thickness of the first and second layers is 35 mm, respectively, and it is pounded to a height of about 50 mm, and the last layer pours all the soil into it, slowly pounding it so that the height of the soil is exactly 80 mm.The three valves were disassembled, the soil sample was removed, and it was placed in a humidifier for maintenance, waiting for the test.

Starch + soil
Weigh a sufficient amount of dry soil that has passed a 2 mm sieve, and then weigh starch according to 1% of the mass of the soil.The starch and soil are slowly mixed to prevent flying dust, and then the total mass of starch and soil is taken as the mass of solid particles, and the water should be added according to the optimal water content (20% + 1%), and the subsequent processing order is as above paragraph.When 3 and 5% starch are added, the treatment method is the same.

Gellan gum + soil
Weigh a sufficient amount of dry soil through a 2 mm sieve, and then weigh powdered gellan glue according to 0.5% of the mass of the soil.The gellan glue and soil are slowly mixed to prevent flying dust, and then the total mass of gellan glue and soil is taken as the mass of solid particles, and the amount of water should be added according to the optimal water content (20% + 1%), and the subsequent treatment order is like plain soil.When 1 and 1.5% gellan glue are added, the processing process is like that of 0.5% gellan glue.

Starch + gellan gum + soil
Weigh a sufficient amount of dry soil through a 2 mm sieve, and then weigh 1% powdered starch and 0.5% powdered gellan glue according to the mass of the soil.The starch, gellan glue, and soil are slowly mixed to prevent flying dust, and then the total mass of starch, gellan glue, and soil is taken as the mass of solid particles, and the amount of water should be added according to the optimal water content (20% + 1%), and the subsequent treatment sequence is the same as mentioned earlier.
The same applies to other combinations.
Pictures for sample preparation and the cylindrical samples for unconfined compressive strength test are shown in Figure 2(d) and (e).

Test scheme
This project mainly studies the effects of starch and gellan gum on the unconfined compressive strength of Shanghai clay.First, for comparison, the unconfined compressive strength test is carried out (ASTM D2166/D2166M-16) on the plain soil, and the processing procedure of the plain soil is exactly the same as when there is admixture.Second, 1, 3, 5% starch and 0.5, 1, 1.5% gellan glue were added separately to Shanghai clay for unconfined compressive strength test.Finally, starch mixed with gellan glue was added to Shanghai clay, and then the unconfined compressive strength test was carried out.The test was divided into four groups: plain soil, starch + soil, gellan gum + soil, and starch + gellan gum + soil.Under different curing age conditions, the unconfined compressive strength test was completed, and each group was repeated in parallel three times to reduce accidental errors, and the total number of samples was 240.The specific number of specimens required for the test is shown in Table 5.
The dosage of starch and gellan gum set in this experiment was as follows: starch: 1, 3, 5% and gellan gum 0.5, 1, 1.5%.The detailed test protocol is shown in Table 6. Figure 7 shows the stress-strain relationship curve of Shanghai clay at the curing age of 7 days and the starch content of 0, 1, 3, and 5%.Since the strength of the sample increases monotonically with age, to save space, only the sample with an age of 7 days is selected to study its strength, which does not affect its regular expression.Figure 7 shows that the axial stress of both plain soil and starch soil first increases and then decreases with the axial strain, and there is a maximum value, and the overall curve shows a "parabola" shape.In addition, the axial stress of starched soil is significantly higher than that of plain soil.Among them, the specimen with a starch content of 3% has the largest peak axial stress.It is explained that starch as an additive could improve the unconfined compressive strength of Shanghai clay.From the perspective of axial strain, in the first half of loading, the stress path at 5% starch was closer to that of plain soil, and the stress path at 1 and 3% starch is significantly higher than that of plain soil.In the second half of loading, the axial stress of plain soil decreases rapidly compared with starch soil, while starch soil decreases slowly, and the decline rate is the slowest at 5% starch.It was shown that the addition of starch to Shanghai clay could effectively control the deformation of the soil, so that the Shanghai clay soil had higher ductility and reduced the brittleness of the soil.
When the curing period was 7 days, the unconfined compressive strength of plain soil was 108.26 kPa, and the unconfined compressive strength of 1, 3, and 5% starch soil was 120.60, 133.25, and 117.61 kPa, respectively, which were increased by 11, 23, and 9% compared with plain soil.This may be related to the gelling of amylose, which condenses soil particles together, increasing the cementation strength between the soil particles.In addition, amylose contains cellulose, which also improves the tensile strength between soil particles.

Effects of curing age on UCS
Figure 8 shows the unconfined compressive strength of each starch content sample at the curing age period 0, 7, 14, 21, and 28 days.Figure 8 shows the unconfined compressive strength of the specimen increases with the curing age, although the rate of increase varies.Moreover, the compressive strength of starch soil was greater than that of plain soil at all ages.When the starch content is 3%, the unconfined compressive strength is the largest, and the increase of 3% starch soil at each age is also the largest, up to 26%.This is followed by 1% starch soil and the lowest is 5%.The increase in the strength of plain soil with the increasing age may be related to the adequate mixing and conservation of soil particles and moisture, or it may be related to the loss of a small amount of moisture.In general, Note: Five samples of plain soil correspond to 5 curing times, and the other groups are the same.the strength of cohesive soils increases with the loss of moisture within certain limits.Although the specimen is placed in a constant temperature and humidity curing box, a small amount of surface moisture will still be dissipated into the air, so its strength will gradually increase, starting from the surface of the soil and gradually developing into the inside of the soil.Therefore, the strength increase in the first 14 days is not obvious, especially in the first 7 days, its strength has hardly increased.After the 14th day, the strength increase accelerates significantly, which may be related to the development of moisture emission into the soil.For starch soils, the increase in strength over time may be related to the development of moisture emission into the soil.The increase in strength of starch soil over time may well also be related to the straight-chain structure of starch colloids, as these straight-chain structures have branching ends (Figure 4) that may penetrate deep into the voids between soil particles, increasing the glue-to-air ratio.In addition, the gradual dissipation of moisture may be another factor in the increase in strength.

Stress-strain curve
For the same reasons as starch soil, the situation at curing age of 7 days was still selected for analysis.Figure 9 shows the stress-strain relationship curve of the specimen when the gellan glue content is 0, 0.5, 1, and 1.5%, respectively.Figure 9 shows that the axial stress of both plain soil and gellan gum soil increases with the increase of axial strain, and when the peak compressive strength is reached, the axial stress decreases with the increase of axial strain.Among them, the peak strength of 0.5 and 1% gellan colloidal (gum) soil was higher than that of plain soil, while the peak strength of 1.5% gellan gum soil was lower than that of primitive (plain) soil.From the perspective of axial strain, in the first half of loading, the stress path of gellan gum soil is closer to that of plain soil, while in the second half, the axial stress of gellan gum soil falls much slower with axial strain than that of primitive soil.It shows that after adding gellan glue to Shanghai clay, its ductility increases significantly, effectively reducing its brittleness, especially when the gellan gum content is 0.5 and 1%.The axial strain corresponding to the peak strength of the plain soil is 2%, and the axial strain corresponding to the peak strength of the gellan glue content of 0.5 and 1% is 2.5 and 3%, respectively, and its ductility is increased by 25 and 50%, respectively.At the curing age of 7 days, the unconfined compressive strength of plain soil was 108.26 kPa, while the compressive strength of 0.5, 1, and 1.5% gellan colloid soil was 119.93, 115.60, and 99.61 kPa, respectively, which were 11, 7, and −8% higher than that of plain soil.Through data analysis, it can be seen that adding an appropriate proportion of gellan glue to Shanghai clay can improve the compressive strength of the soil, but excessive gellan glue will reduce the strength of the sample.On the one hand, from the molecular structure of gellan gum, it contains branch ends (Figure 5), which extend to the void between soil particles, increasing the glue-to-air ratio and thus improving the strength.On the other hand, because the gelling of gellan glue is very strong, it can form a gel at a  content of 0.05%, so too much gellan glue may form a continuous lumpy gellan colloid in the soil, rather than evenly dispersed into the soil, so its strength may decrease.

Effect of curing age on peak strength of gellan gum soil
Figure 10a and b shows the unconfined compressive strength of plain soil and 0.5, 1, and 1.5% gellan colloidal soil at a curing age of 0, 7, 14, 21, and 28 days.Figure 10 shows that the unconfined compressive strength of both plain soil and gellan colloidal soil increases with the increase of curing age, and the compressive strength of 0.5 and 1% gellan gum soil is greater than that of primitive soil, while the compressive strength of 1.5% gellan gum soil is lower than that of primitive soil.At all ages, 0.5% gellan colloidal soil has the largest unconfined compressive strength.Compared with the curing age of 0 days, the strength of 0, 0.5, 1, and 1.5% gellan gum soil at 28 days was increased by 7, 11, 12, and 22%, respectively.It can be seen that when the content of gellan gum is 1.5%, although its peak strength is lower than that of plain soil at all ages, it increases the fastest with time.The reason is that the polymer substances in the gellan glue gradually form a cross-linked structure with time after curing, so that the gellan glue is completely cured and the strength increases rapidly, so that its strength almost catches up with the strength of plain soil at the 28th day.In summary, 0.5% gellan colloidal soil has the highest compressive strength.However, compared with monodoped starch, its strength effect is lower than that of starch soil.However, the ductility of gellan gum is much higher than that of starch soil, which may be caused by the gelability of gellan gum being higher than that of cornstarch.

The combined effect of starch and gellan
gum on UCS

Stress-strain curve
Figure 11 shows the unconfined compressive strength of the specimen after mixing 0.5, 1, and 1.5% gellan glue in 1, 3, and 5% starch at the 7 days curing age.It can be seen that the peak strength order of each combination is 5% starch + 0.5% gellan gum > 5% starch + 1.0% gellan gum > 5% starch + 1.5% gellan gum > single doped 5% starch + 0% gellan gum > 0% starch + 0% gellan gum.In this test, when 5% starch mixed with 0.5% gellan glue at the age of 7 days, the unconfined compressive strength reached the maximum, and the maximum value was 142.58 kPa.Compared to plain soil, this is an increase of 32%.It can also be seen in the figure that as long as the sample contains gellan glue, its failure strain is greatly increased.When starch 5% is mixed with 0.5% gellan glue, not only its peak strength reaches the maximum but also its failure strain increases significantly, and compared with plain soil (its failure strain is 1.9%), its failure strain reaches 3.3%, which is 74% higher than that of primitive soil.Once again, gellan glue can indeed improve the ductility of the sample and reduce its brittleness, which is very beneficial to engineering construction because it has a large deformation before failure and also provides a longer buffer.However, for soil samples with only 5% starch, the failure strain is even lower than that of plain soil, and although after reaching peak strength, the strength decline is still slow and lower than that of plain soil.
Figure 12 shows the histogram of the peak compressive strength of 0.5, 1, and 1.5% gellan colloidal soil mixed with 1, 3, and 5% starch at a curing age of 7 days.
Figure 12 shows that the strength value of the specimen decreases with the increase of the gellan gum content (0.5, 1, and 1.5%).Except for cases where the starch content is 0%, the strength is maximum when the gellan gum content is 0.5%, regardless of the starch content.
When 1% starch is mixed with 0.5, 1, and 1.5% gellan glue, the peak compressive strength decreases with the increase of the gellan glue content, which is increased by 4, −7, and −23% compared with plain soil, and −7, −17, and −31%, respectively, compared with single 1% starch.It is explained that within the scope of this test, as the gellan glue is continuously added on the basis of 1% starch content, the compressive strength of the soil is continuously reduced, which shows that too low of the starch content will result in the gellan glue not playing a corresponding reinforcement role on the soil.The peak compressive strength of 3% starch mixed with 0.5, 1, and 1.5% gellan gum decreased with the increase of gellan glue, which increased by 22, 9, and 2% compared with plain soil, and increased by 6, −5, and −12%, respectively, compared with single 3% starch, indicating that within the scope of this test, the content of gellan glue was too high to make the compressive strength of the soil lower than that of mono-  doped starch soil.The peak compressive strength of 5% starch mixed with 0.5, 1, and 1.5% gellan gum decreased with the increase of gellan glue, which was increased by 32, 24, and 8% compared with plain soil, and increased by 27, 19, and 4%, respectively, compared with single 5% starch.The compressive strength of 5% starch mixed with gellan colloidal soil was higher than that of plain soil and also higher than that of single 5% starch.It shows that within the scope of this test, as the gellan glue is continuously added on the basis of the 5% starch content, the reinforcement effect is better than that of single-doped starch soil though the compressive strength of the soil is continuously reduced.
In summary, with the addition of starch and gellan gum to Shanghai clay at the same time, although the strength changes greatly, its ductility is greatly improved compared with that of plain soil.Although single-doped starch and single-doped gellan glue can play a reinforcing role in Shanghai clay, when the two admixtures are added at the same time, the appropriate ratio should be selected to play a good reinforcement effect.Generally speaking, when the content of gellan gum is too high, the improvement of the compressive strength of the soil will be inhibited, which is mostly lower than that of mono-doped starch.In the test range, the optimal combination of stiffening rate was 5% starch and 0.5% gellan gum, and its compressive strength was increased by 32% compared with plain soil and 27% compared with single-doped 5% starch, and the ductility was greatly improved compared with plain soil, single-doped starch, and gellan gum-reinforced soil at this reinforcement rate.

Effect of curing age
Figure 13 shows the unconfined compressive strength diagram of 0.5, 1.0, and 1.5% gellan glue after 1, 3, and 5% starch is added to plain soil at a curing age of 0, 7, 14, 21, and 28 days.Figure 13 shows that with the extension of the curing period, the unconfined compressive strength of plain soil, mono-doped starch soil, and starch-mixed gellan colloidal soil gradually increased.Figure 13(a) shows that the maximum compressive strength occurs when the single 1% starch is doped and the age is at day 28. Figure 13(b) shows that 3% starch mixed with 0.5% gellan colloidal soil has the highest peak strength at 28 days of age, reaching 163 kPa.The strength of 3% starch mixed with 1 and 1.5% gellan gum at all ages was lower than that of single 3% starch soil.Figure 13(c) shows that its maximum strength occurs when 3% starch mixed with 0.5% gellan gum and the age is 28 days, reaching 160 kPa.It can be seen that although the strength of 5% starch + 0.5% gellan glue specimen is greater than 3% starch + 0.5% gellan gum at an age of 7 days, the strength of 3% starch + 0.5% gellan glue specimen has exceeded 5% starch + 0.5% gellan gum at an age of 28 days.It can be seen that age is also an important influencing factor.

Regression analysis
To avoid duplicate analysis, in this article, the unconfined compressive strength at 14 days was selected for multiple regression analysis.The contents of starch (x) and gellan gum (y) were selected as the independent variables, and the unconfined compressive strength was the dependent variable (z), and the nonlinear surface was fitted by Origin2017 software, and the functional relationship was as follows: The regression results are as follows: (2) This is shown in Figure 14.R 2 = 0.90682 of the regression model and the adjusted R 2 = 0.88523 indicate that the 89% change of unconfined compressive strength can be explained by starch content and gellan gum content, and the fitting is very successful.In the future, as long as the content of starch and gellan gum is known, the unconfined compressive strength at the age of 14 days can be directly calculated through the theoretical formula, saving the trouble of testing, which provides great convenience for future engineering survey, design, and construction.It can be seen from the regression result (2) that generally, the unconfined compressive strength (UCS) is 113 kPa as the contents of starch and gellan gum are all zero.And the correlation of gellan gum with UCS is stronger than with starch due to the coefficient of gellan gum being larger than that of starch.Note that this regression result is applicable to the condition that the dimension of UCS is kPa, starch and gellan gum is percent (%), and the curing time is 14 days.The adjusted coefficient of determination is approximately 89%, meaning that the 89% change of UCS can be explained by the change of starch and gellan gum, the remaining change of 11% depending on factor of random variation.
The use of starch as a treatment agent has the advantages of natural, environmentally friendly, nontoxic, easy to biodegrade, etc., and will not have a negative impact on the environment.Gellan gum can be dissolved in cold water to form a gel, which has good stability, acid resistance, high temperature resistance, and resistance to microorganisms and enzymes.The use of the two for highway, slope, foundation, and slope reinforcement has originality and has a wide range of application prospects.Since the strength extreme points of the admixture combination in this test all appeared at 0.5% gellan glue, it is speculated that there may still be extreme points when the gellan gum content is less than 0.5%.According to the characteristics of gellan glue, that is, the gellan glue content has strong gelability when it is very low (0.05%), and the possibility of this situation is very large.The lowest point of the gellan gum content in this test is set at 0.5%, which is the deficiency of this test, which should become the direction of future research.In addition, the influence of age on strength cannot be ignored.Although the strength of 5% starch + 0.5% gellan gum specimen was greater than 3% starch + 0.5% gellan gum at the age of 7 days, the strength of 3% starch + 0.5% gellan gum specimen exceeded 5% starch + 0.5% gellan gum at 28 days of age.How the strength will change after 28 days is still unknown, which will be another direction to explore.The accessibility, cheapness, and local availability of starch and gellan gum make possible their application in ground improvement, slope stability, civil engineering design, etc.In addition, the two materials are environmentally friendly and have no negative impact on environment, which are completely in line with the concept of the sustainable and green development currently and worldwide, having a wide range of application prospects.

SEM analysis
Figure 15 shows part of images from the analysis of scanning electron microscope (SEM) for samples of 7 days curing time after unconfined compressive test.
It can be obviously seen from the image that there is a bigger gap between the plain soil particles and the binding is looser than those of treated soil.There is much more lamellar cementation in image (2)c than in image (2)b, maybe meaning that gellan gum is prone to forming mesh cementation, while the starch is cemented around the soil particles.There seems more dense cementation in image (3) b than in image (3)c, meaning that the soil treated only by starch tends to have high strength than that treated only by gellan gum.The difference between diagram series (4) and (1) is magnification.The diagram series (1) is magnified by 500 times, while diagram series (4) is magnified by 10000 times that make us look inside and particulate cementation status from a more microscopic perspective.There is similar situation in diagram series (4).The compactness of particle in image (4)d seems to surpass those of in image (4)b and in image (4)c, demonstrating that the soil treated by starch + gellan gum is superior to those treated by starch or gellan gum separately in terms of strength.There is similar situation in diagram series (1).

Conclusions
1.The addition of starch to the sample alone can improve the unconfined compressive strength of the soil, and when the starch content is 3%, the unconfined compressive strength reaches the maximum.2. The addition of gellan glue alone to the sample can improve the unconfined compressive strength of the soil, and when the gellan glue content is 0.5%, the unconfined compressive strength reaches the maximum within the scope of the test.
3. At the age of 7 days, the unconfined compressive strength of the soil body was the largest when 5% starch mixed with 0.5% gellan glue, but when the age was 28 days, the strength of 3% starch mixed with 0.5% gellan glue was the largest.4. Whether it is mixed with starch or gellan gum, its strength increases with curing age. 5.After regression analysis, the function relationship between the unconfined compressive strength of the sample at the age of 14 days and starch and gellan gum content was obtained.

Figure 1 :
Figure 1: Geographic site for collecting of soil.

Figure 7 :
Figure 7: Effect of starch on stress-strain curve of Shanghai clay.

Figure 8 :
Figure 8: Effect of curing age on the unconfined compressive strength of starch soil.

Figure 9 :
Figure 9: Effect of gellan gum on stress-strain curve of Shanghai clay.

Figure 10 :
Figure 10: Effect of curing age on peak strength of gellan colloidal soil.(a) Curve graph and (b) histogram.

Table 2 :
The chemical composition of the test soil * SiO 2 Al 2 O 3 Fe 2 O 3 CaO K 2 O MgO Na 2 O TiO 2 Others

Table 3 :
Composition and content of corn starch for test (per 100 g) * *Note: Provided by Shanghai Zhongzheng Analysis and Test Technology Service Center.

Table 4 :
Composition and properties of gellan gum for testing * *Note: Provided by Shanghai Zhongzheng Analysis and Test Technology Service Center.

Table 5 :
Total number of specimens

Table 6 :
Detailed test protocol